Polysilicon surface micromachining adapts planar fabrication process steps known to the integrated circuit (IC) industry to manufacture microelectromechanical or micromechanical devices. The standard building-block processes for polysilicon surface micromachining are deposition and photolithographic patterning of alternate layers of low-stress polycrystalline silicon (also termed polysilicon) and a sacrificial material (e.g. silicon dioxide or a silicate glass). Vias etched through the sacrificial layers at predetermined locations provide anchor points to a substrate and for mechanical and electrical interconnections between the polysilicon layers. Functional elements of the device are built up layer by layer using a series of deposition and patterning process steps. After the device structure is completed, it can be released for movement by removing the sacrificial material in part or entirely by exposure to a selective etchant such as hydrofluoric acid (HF) which does not substantially attack the polysilicon layers.
The result is a construction system generally consisting of a first layer of polysilicon which provides electrical interconnections and/or a voltage reference plane (e.g. a ground plane), and up to three or more additional layers of mechanical polysilicon which can be used to form functional elements ranging from simple cantilevered beams to complex systems such as an electrostatic motor connected to a gear train. Typical in-plane lateral dimensions of the functional elements can range from one micron to several hundred microns or more, while individual layer thicknesses are typically about 1-3 microns. Because the entire process is based on standard IC fabrication technology, a large number of fully assembled devices can be batch-fabricated on a silicon substrate without any need for piece-part assembly.
For various types of MEM devices, a precise control over movement or positioning is needed. Such precise movement control is difficult using present MEM motors or microengines (see e.g. U.S. Pat. No. 5,631,514 to Garcia et al which discloses a MEM engine which rotates a gear in substantially 90.degree. increments and requires multiple complex drive signals).
The use of a reciprocating shuttle to form a wedge-type stepping motor as disclosed in U.S. Pat. No. 5,959,376 to Allen provides an improvement in precise positioning of a gear. However, the reciprocating shuttle produces an unbalanced actuation force on a hub about which the gear rotates, thereby limiting the durability and reliability of such a device.
An advantage of the present invention is a MEM apparatus is provided which provides a precise open-loop positioning of a moveable member such as a ring gear, a stage, or a rack by using a ratcheting mechanism.
Another advantage of the present invention is that a rotary MEM apparatus formed according to the present invention has actuation forces that are substantially balanced to minimize wear and thereby improve reliability.
Yet another advantage is that manufacturing tolerances (e.g. due to photomask misalignment) can be less critical compared to other MEM devices since motion of a majority of the elements in the MEM actuator of the present invention is limited to small angles and distances rather than requiring full rotation about an axis.
Another advantage of the present invention is that the MEM apparatus operates with simple drive signals, with the rotation or translation of a moveable member being precisely determinable from the drive signals.
Still another advantage of the present invention is that a relatively high torque can be provided to a moveable member (e.g. a gear, stage, or rack) without the need for any additional gears.
A further advantage of the present invention is that an electrostatic or thermal actuator for driving the moveable member can be located within an outline of the member so that no additional space is required on a substrate.
These and other advantages of the method of the present invention will become evident to those skilled in the art.